This disclosure relates to an image forming apparatus updating a gamma correction table for correcting input/output characteristics of a printing device, and a non-transitory computer-readable recording medium including a calibration program.
A typical image forming apparatus is known, which executes calibration for making the input/output characteristics of the printing device equal to desired input/output characteristics. The typical image forming apparatus updates the gamma correction table to thereby execute the calibration.
In a case where the image forming apparatus includes an electrophotographic printer as the printing device, as a result of a temperature or humidity change, the input/output characteristics of the printing device may no longer be equal to the desired input/output characteristics. More specifically, an amount of toner adhering to a recording medium may change. Long-term continuous execution of printing by the image forming apparatus may, for example, cause changes in temperatures of various sections of the printer. Therefore, as a result of the long-term continuous execution of printing, the input/output characteristics of the printing device may no longer be equal to the desired input/output characteristics. As a result of long-term use of the image forming apparatus, characteristics of the various sections of the printer may change. For example, the charging characteristics of a photosensitive drum may change. Therefore, as a result of the long-term use of the image forming apparatus, the input/output characteristics of the printing device may no longer be equal to the desired input/output characteristics.
Therefore, in a case where one of parameters such as humidity, temperature, and the number of prints has satisfied corresponding one of given conditions respectively set for these parameters, the image forming apparatus executes the calibration. The typical image forming apparatus executes the calibration as shown in FIG. 13.
FIG. 13 is a flowchart of an operation performed by the image forming apparatus at time of the execution of the calibration.
As shown in FIG. 13, the image forming apparatus renders a patch pattern, which is formed of any given screen, on a recording medium by the printer (step S51). The patch pattern includes a plurality of patch images.
Next, the image forming apparatus measures respective concentration values of the plurality of patch images included in the patch pattern rendered on the recording medium in step S51 (step S52). More specifically, the image forming apparatus individually reads the patch images with a photo sensor such as a reflection densitometer. The image forming apparatus measures the concentration value of each patch image based on a corresponding output of the photo sensor.
FIG. 5 is a diagram illustrating one example of the input/output characteristics of the printer. In FIG. 5, a horizontal axis represents input values and a vertical axis represents concentration values. FIG. 5 shows correspondence relationship between the input values and the concentration values measured in step S52. In other words, FIG. 5 shows the correspondence relationship between the input values and the concentration values measured by use of the photo sensor.
As shown in FIG. 5, the measured concentration values may include any concentration value exceeding “Dmax” in some cases. The “Dmax” is a designed maximum concentration value of the image forming apparatus. In FIG. 5, “Dmin” is a designed minimum concentration value of the image forming apparatus. In FIG. 5, each open circle is plotted to indicate the concentration value of each patch image measured in step S52. That is, each open circle denotes a sampling point.
As shown in FIG. 13, the image forming apparatus starts a loop process after the process of step S52 (step S53). More specifically, the image forming apparatus individually repeats the processes of steps S54 to S56 for all the concentration values of the patch images measured in step S52. In other words, the image forming apparatus repeats the processes of steps S54 S56 for each input value.
In step S54, the image forming apparatus normalizes the concentration value measured in step S52 by formula (1) below. In formula (1), “Din” is the concentration value measured in step S52. “Dout” is a concentration value obtained by normalizing the concentration value Din. “N” is a designed maximum tone value of the image forming apparatus. The maximum tone value N is, for example, 255.Dout=(Din−Dmin)/(Dmax−Dmin)×N  (1)
In step S55, the image forming apparatus judges whether or not the concentration value Dout generated in step S54 exceeds the maximum tone value N.
As a result of judgment that the concentration value Dout exceeds the maximum tone value N (Yes in step S55), the image forming apparatus converts the concentration value Dout into “N” (step S56). In other words, in a case where the concentration value Din measured in step S52 exceeds the designed maximum concentration value Dmax of the image forming apparatus, the image forming apparatus converts the concentration value Din into the maximum concentration value Dmax.
FIG. 14 is a diagram obtained by normalizing the input/output characteristics shown in FIG. 5. In FIG. 14, a horizontal axis represents input values and a vertical axis represents concentration values.
As shown in FIG. 14, of outputs of the normalized input/output characteristics, any output exceeding the maximum tone value N is converted into “N”. Hereinafter, the outputs of the normalized input/output characteristics may be described as outputs obtained by the normalization.
As shown in FIG. 13, as a result of ending of the loop process (step S59), the image forming apparatus executes interpolation in a manner such that the outputs obtained by the normalization are smoothly connected together (step S60). More specifically, the image forming apparatus interpolates between the sampling points obtained by the normalization.
FIG. 15 shows a graph obtained through the interpolation executed on the input/output characteristics shown in FIG. 14. In FIG. 15, a horizontal axis represents input values and a vertical axis represents concentration values.
As shown in FIG. 15, as a result of the interpolation executed in step S60, the sampling points are arranged in a manner such as to be smoothly connected together.
As shown in FIG. 13, after the process of step S60, the image forming apparatus generates a gamma correction table (step S61). The gamma correction table is generated based on the graph generated in step S60 and the desired input/output characteristics.
FIG. 8 is a diagram illustrating one example of the desired input/output characteristics. In FIG. 8, a horizontal axis represents input values and a vertical axis represents concentration values. FIG. 16 is a diagram illustrating the gamma correction table generated from the graph shown in FIG. 15. In FIG. 16, a horizontal axis represents input values and a vertical axis represents input values obtained through gamma correction.
With the desired input/output characteristics shown in FIG. 8, the input values and the output values are in a directly proportional relationship. In this case, the image forming apparatus generates a graph line-symmetric to the graph shown in FIG. 15 with the graph shown in FIG. 8 as a symmetric axis. The graph line-symmetric to the graph shown in FIG. 15 is the gamma correction table shown in FIG. 16. The graph shown in FIG. 16 is a graph with a function inverse to that of the graph shown in FIG. 15.
As shown in FIG. 13, after the process in step S61, the image forming apparatus updates a gamma correction table corresponding to the screen used in step S51 to the gamma correction table generated in step S61 (step S62). Operation shown in FIG. 13 ends as a result of the update of the gamma correction table.
As a result of gamma correction of the input/output characteristics shown in FIG. 5 with reference to the gamma correction table shown in FIG. 16, by the image forming apparatus, the input/output characteristics shown in FIG. 5 are changed to input/output characteristics shown in FIG. 17. In FIG. 17, a horizontal axis represents input values and a vertical axis represents concentration values.